The present invention relates to a synthetic quartz glass powder suited for use as a raw material for ultra-high purity quartz glass products used in the field of semiconductor production, especially in the high temperature region of not lower than 1,000.degree. C.
Crucibles used for producing semiconductor single crystals and jigs have been manufactured by powdering natural quartz and fusing the powder. Natural quartz, even high-quality, contains various metallic impurities and is not entirely satisfactory in respect of purity. If metallic impurities contaminate high-purity single crystals, whose importance and demand are increasing with the recent trend toward higher performance in the semiconductor industries, the performance of the semiconductors produced therefrom is adversely affected to an objectionable degree, so that it is unacceptable to use crucibles or jigs which have possible contamination with metallic impurities. Thus, high-purity quartz glass powder produced by synthesis is required.
Recently, quartz glass produced by the sol-gel method using alkoxysilane as a raw material has been proposed as a high-purity silicate source. For instance, Japanese Patent Application Laid-Open (KOKAI) No. 62-176928 (1987) discloses a method for producing quartz (silica) glass powder by hydrolyzing alkoxysilane at a specified pH to prepare a gel, powdering the gel, and after drying, calcining the powder.
In the production of synthetic quartz glass powder by the sol-gel method, alkoxysilane used as a raw material is first hydrolyzed and polycondensed to form a wet gel and then the by-product alcohols and water are removed by drying to obtain a dry gel. In the above reaction, however, 100% of the alkoxy groups are not reacted and some of them remain in the dry gel as bound alkoxy groups. Also, the alcohols generated as by-product of the reaction are partly left in the dry gel. In fact, determinations show that carbon concentration in the dry gel is 1,000 to 10,000 ppm.
When the dry gel is calcined, carbon is mostly burned away but some may remain unburned and such unburned carbon may be trapped in glass to appear as black contaminant. When such black contaminant intermixes in a synthetic quartz powder product, it may give rise to CO or CO.sub.2 gas causing foaming when the powder product is fused for molding into a crucible or an ingot. Attempts have been made to prevent formation of such black contaminant; for instance, the dry gel has been maintained at a temperature in the range of 400.degree.-600.degree. C. before the pores in the dry gel are closed so as to remove carbon from the dry gel. These attempts have not been entirely satisfactory and do not completely prevent formation of black contaminant. The thus produced foam-containing quartz crucibles or furnace core tubes had the various problems such as deterioration of dimensional stability in use at a high temperature and bursting of the foams during lift-up of single crystals to cause undulation of liquid level and crystal defects.
As a result of the present inventors' studies, it has been found that when producing of synthetic quartz glass powder by the sol-gel method, when at least a part of the calcining step at a temperature of not lower than 1,000.degree. C. is conducted in an atmosphere with an oxygen concentration of not less than 30 vol %, it is possible to obtain a synthetic quartz glass powder in which formation of black contaminant is substantially inhibited during production. This synthetic quartz glass, therefore, forms few foams during fusion molding.